The present invention is directed toward an improved method for recovering a hydrogen-rich gas stream from a hydrogen and hydrocarbon effluent of a catalytic hydrocarbon conversion zone. More particularly the described inventive technique is adaptable for utilization in catalytic hydrocarbon conversion reactions which result in a net production of hydrogen.
Various types of catalytic hydrocarbon conversion reaction systems have found widespread utilization throughout the petroleum and petrochemical industries for effecting the conversion of hydrocarbons to a multitudinous number of products. The reactions employed in such systems are either exothermic or endothermic, and of more importance to the present invention, often result in either the net production of hydrogen or the net consumption of hydrogen. Such reaction systems, as applied to petroleum refining, have been employed to effect numerous hydrocarbon conversion reactions including those which predominate in catalytic reforming, ethylbenzene dehydrogenation to styrene, propane and butane dehydrogenation, etc.
Petroleum refineries and petrochemical complexes customarily comprise numerous reaction systems. Some systems will be net consumers of hydrogen while other systems within the refinery or petrochemical complex may result in the net production of hydrogen. Because hydrogen is a relatively expensive item, it has become the practice within the art of hydrocarbon conversion to supply hydrogen from reaction systems which result in the net production of hydrogen to reaction systems which are net consumers of hydrogen. Occasionally the net hydrogen being passed to the net hydrogen-consuming reaction systems must be of high purity due to the reaction conditions and/or the catalyst employed in the systems. Such a situation may require treatment of the hydrogen from the net hydrogen-producing reaction systems to remove hydrogen sulfide, light hydrocarbons, etc., from the net hydrogen stream.
Alternatively, the hydrogen balance for the petroleum refinery or petrochemical complex may result in excess hydrogen, i.e., the net hydrogen-producing reaction systems produce more hydrogen than is necessary for the net hydrogen-consuming reaction systems. In such an event the excess hydrogen may be sent to the petroleum refinery or petrochemical complex fuel system. However, because the excess hydrogen often has admixed therewith valuable components, such as C.sub.3.sup.+ hydrocarbons, it is frequently desirable to treat the excess hydrogen to recover these components prior to its passage to fuel.
Typical of the net hydrogen-producing hydrocarbon reaction systems are catalytic reforming, catalytic dehydrogenation of alkyl-aromatics and catalytic dehydrogenation of paraffins. Commonly employed net hydrogen-consuming reaction systems are hydrotreating, hydrocracking and catalytic hydrogenation. Of the above mentioned net hydrogen-producing and consuming hydrocarbon reaction systems, catalytic reforming ranks as one of the most widely employed. By virtue of its wide application and its utilization as a primary source of hydrogen for the net hydrogen-consuming reaction systems, catalytic reforming has become well known in the art of hydrocarbon conversion reaction systems. Accordingly the following discussion of the invention will be in reference to its application to a catalytic reforming reaction system. However, the following discussion should not be considered as unduly limiting the broad scope of the invention which has wide application in many hydrocarbon conversion reaction systems. Those having ordinary skill in the art will well recognize the broad application of the present invention and the following will enable them to apply the invention in all its multitudinous embodiments.
It is well known that high quality petroleum products in the gasoline boiling range including, for example, aromatic hydrocarbons such as benzene, toluene and the xylenes, are produced by the catalytic reforming process wherein a naphtha fraction is passed to a reaction zone wherein it is contacted with a platinum-containing catalyst in the presence of hydrogen. Generally, the catalytic reforming reaction zone effluent, comprising gasoline boiling range hydrocarbons and hydrogen, is passed to a vapor-liquid equilibrium separation zone and is therein separated into a hydrogen-containing vapor phase and an unstabilized hydrocarbon liquid phase. A portion of the hydrogen-containing vapor phase may be recycled to the reaction zone. The remaining hydrogen-containing vapor phase is available for use either by the net hydrogen-consuming processes or as fuel for the petroleum refinery or petrochemical complex fuel system. While a considerable portion of the hydrogen-containing vapor phase is required for recycle purposes, a substantial net excess is available for other uses.
Because the dehydrogenation of naphthenic hydrocarbons is one of the predominant reactions of the reforming process, substantial amounts of hydrogen are generated within the catalytic reforming reaction zone. Accordingly a net excess of hydrogen is available for use as fuel or for use in a net hydrogen-consuming process such as the hydrotreating of sulfur-containing petroleum feedstocks. However, catalytic reforming also involves a hydrocracking function among the products of which are relatively low molecular weight hydrocarbons including methane, ethane, propane, butanes and the pentanes, substantial amounts of which appear in the hydrogen-containing vapor phase separated from the reforming reaction zone effluent. These normally gaseous hydrocarbons have the effect of lowering the hydrogen purity of the hydrogen-containing vapor phase to the extent that purification is often required before the hydrogen is suitable for other uses. Moreover, if the net excess hydrogen is intended for use as fuel in the refinery or petrochemical complex fuel system, it is frequently desirable to maximize the recovery of C.sub.3.sup.+ hydrocarbons which are valuable as feedstock for other processes. It is therefore advantageous to devise a method of purifying the hydrogen-containing vapor phase to produce a hydrogen-rich gas stream and to recover valuable components such as C.sub.3.sup.+ hydrocarbons.